Carbonyl iron particles (CIP iron) mixed into a matrix of polymer or paint are known for use as electromagnetic interference (EMI) shielding. However, these structures do not have optimized magnetic properties because the magnetic properties cannot be tuned for application-specific needs.
Other known structures include perpendicularly aligned microwires and/or nanowires encapsulated in polymer matrices. Examples include carbon nanotubes in polymer gaskets and silicon microwires in polymer sheets (J. A. Beardslee, Magnetic alignment of high-aspect ratio microwires into perpendicular arrays, Ph. D. Thesis, California Institute of Technology, 2014). However, these structures are not magnetic.
Another example includes iron nanowires in polyaniline (H. Cao, Z. Xu, D. Sheng, J. Hong, H. Sang, Y. Du, J. Mater. Chem., 11, 958-960 (2001)). However, neither iron nor polyaniline are air stable materials and may fail after environmental exposure in most applications. This precludes their use for EMI and antenna shielding.
Known structures also include foil substrates with surfaces containing perpendicularly aligned arrays of iron oxide (Fe2O3) nanowires [C. H. Kim, H. J. Chun, D. S. Kim, S. Y. Kim, J. Park, J. Y. Moon, G. Lee, J. Yoon, Y. N Jo, M. H. Jung, S. I. Jung, C. J. Lee, Appl. Phys. Lett. 89, 223103 (2006); P. Hiralal, H. E. Unalan, K. G. UWijayantha, A. Kursumovic, D. Jefferson, J. L. MacManus-Driscoll, G. A. J. Amaratunga, Nanotechnology 19, 455608 (2008)] and iron oxide nanowires converted into magnetic cobalt ferrite (CoFe2O4) [C. H. Kim, Y. Myung, Y. J. Cho, H. S. Kim, S.-H. Park, J. Park, J.-Y. Kim, B. Kim, J. Phys. Chem. C 113, 7085 (2009)]. Although the wires in the latter paper form magnetic ferrite microwire arrays with template-free growth at densities of approximately 1 wire/μm2, the wires are chemically bonded to a foil substrate from which the wires were initially grown. The nanowire arrays are anchored to the foil substrate surface and lack a flexible polymer matrix. Thus, the arrays cannot conform to a complex surface and lack abrasion resistance to removal from the foil substrate.
Furthermore, known structures containing microwires or nanowires encapsulated in a flexible polymer layer use aligned wires at low densities, approximately 1 wire/10 μm2 (J. A. Beardslee, Magnetic alignment of high-aspect ratio microwires into perpendicular arrays, Ph. D. Thesis, California Institute of Technology, 2014). These structures are made by physically inserting fully formed wires into aligning templates etched into a substrate such as silicon. After assembly, the wires are encapsulated in a flexible polymer layer and lifted off of the aligning substrate. However, it is not known how to increase the areal density of the aligned wires.
Lastly, arrays at much higher wire densities, often greater than 1 wire/μm2, have been formed by directly growing wire arrays within templates such as anodic porous alumina. (J. A. Beardslee, Magnetic alignment of high-aspect ratio microwires into perpendicular arrays, Ph. D. Thesis, California Institute of Technology, 2014). After growth, the alumina template is dissolved and the wires are encapsulated in a flexible polymer layer. This technique would not work for magnetic ferrites because the same solution that dissolves the anodic alumina would also dissolve the ferrites. Further, these wire arrays do not have the crystal quality characteristic of template free wire growth and it is not known how to achieve the crystal quality of template-free growth while still using a template.
As appreciated from the deficiencies of previous efforts described above, a method for forming magnetic microwires aligned in a non-conductive, flexible polymer matrix would be beneficial.